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LTC4265

4265f

APPLICATIONS INFORMATION

SIGNATURE CORRUPT OPTION

In some designs that include an auxiliary power option,

it is necessary to prevent a PD from being detected by a

PSE. The LTC4265 signature resistance can be corrupted

with the SHDN pin (Figure 3). Taking the SHDN pin high

will reduce the signature resistor below 11k which is an

invalid signature per the IEEE 802.3af/at speciﬁcation, and

alerts the PSE not to apply power. Invoking the SHDN pin

also ceases operation for classiﬁcation and disconnects

the LTC4265 load from the PD input. If this feature is not

used, connect SHDN to VIN.

During classiﬁcation probing, the PSE presents a ﬁxed

voltage between 15.5V and 20.5V to the PD (Figure 2).

The LTC4265 asserts a load current representing the PD

power classiﬁcation. The classiﬁcation load current is

programmed with a resistor RCLASS that is chosen from

Table 2.

Table 2. Summary of Power Classiﬁcations and LTC4265

RCLASS Resistor Selection

CLASS

USAGE

MAXIMUM

POWER LEVELS

AT INPUT OF PD

(W)

NOMINAL

CLASSIFICATION

LOAD CURRENT

(mA)

LTC4265

RCLASS

RESISTOR

(Ω, 1%)

Default

0.44 to 12.95

< 0.4

Open

Optional

0.44 to 3.84

10.5

124

Optional

3.84 to 6.49

18.5

69.8

Optional

6.49 to 12.95

45.3

Optional

12.95 to 25.5

30.9

2-EVENT CLASSIFICATION AND THE T2PSE PIN

A Type-2 PSE may declare the availability of high power by

performing 2-event classiﬁcation (Layer 1) or by commu-

nicating over the high speed data line (Layer 2). A Type-2

PD must recognize both layers of communication. Since

Layer 2 communications takes place directly between the

PSE and the LTC4265 load, the LTC4265 concerns itself

only with recognizing 2-event classiﬁcation.

Figure 3. 25k Signature Resistor with Disable

GND

VIN

SHDN

LTC4265

TO AUX

4265 F03

25k SIGNATURE

RESISTOR

14k

PSE

CLASSIFICATION

Classiﬁcation provides a method for more efﬁcient power

allocation by allowing the PSE to identify a PD power clas-

siﬁcation. Class 0 is included in the IEEE speciﬁcation for

PDs that don’t support classiﬁcation. Class 1-3 partitions

PDs into 3 distinct power ranges. Class 4 includes the new

power range under IEEE802.3at (See Table 2).

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